When it comes to the deadliest voltage there really is no single deadliest voltage that can kill you.
In reality it's actually the current "amperes" or amperage that flows through the body that kills and not the voltage itself.
The amperes or amperage determines how severe the shock from the electricity is and the potential for death.
The voltage is the potential difference and you can think of the voltage as being the push or pressure which drives the electricity through a circuit.
The current is the flow of the electricity.
The current is the actual flow of electrical charge that is measured in amperes.
Current is what matters when it comes to the electric shock being deadly as higher current that flows through the body can disrupt vital functions of the body which include heart rhythm and can cause severe burns.
While any current flowing through the body can become lethal it's mainly currents between 100 milliamperes and 200 milliamperes and above to be considered lethal.
Having a higher voltage source can also drive a higher current through your body although the current itself is the primary factor when it comes to the severity of the electric shock.
Even ordinary household voltage of 110 volts to 240 volts can be deadly if the current flowing through the body is high enough.
And even low voltages as low as 42 volts have caused death.
I've touched 240 volts and 110 volts by accident and I was able to let go quickly but some people may not be able to let go of the 240 volts so it can vary.
When working on electricity be sure the power is off to avoid any electric shock and lock the breaker box out to prevent the power being turned back on.
The route that the current also takes through your body also significantly impacts the severity of the shock.